TIP: Although all examples from this documentation are instantiating each class, a single point of entry is also available:
$bitbucket = new \Bitbucket\API\Api();
$bitbucket->setCredentials(new Http\Message\Authentication\BasicAuth('username', 'password'));
/** @var \Bitbucket\API\User $user */
$user = $bitbucket->api('User');
/** @var \Bitbucket\API\Repositories\Issues $issues */
$issues = $bitbucket->api('Repositories\Issues');